In recent years, in order to reduce the energy costs, water system of reducing the amount of blow-down water system and carrying out the operation in a high concentration one increasing. In such water systems, a scale component dissolved in water, such as calcium, magnesium, silica, etc., also highly in concentrated, and hence, there is such a concern that such a component form scale deposition, thereby causing a reduction of thermal efficiency or clogging of a heat exchanger, or the like.
In particular, in boiler water systems, scale components which have been brought into the inside of a boiler vessel, such as calcium, magnesium, silica, iron, etc., form scale deposition on heating surfaces of high heat load, thereby possibly causing bulging, creeping, rupture, or a reduction of thermal efficiency due to overheating of steel materials.
In addition, the adhesion of a scale onto the heating surface causes heat transfer disturbance and generates an energy loss, resulting in an increase of the cost of fuel. For this reason, in the boiler water system and so on, in order to prevent the adhesion of a scale, water softened by removing calcium and magnesium that are the hard components in the raw water by a water softener is used as the feed-water.
In addition, there is also adopted a water treatment method in which a scale dispersant is added into the boiler water, thereby not only inhibiting the deposition of a trace amount of a hard component or a scale component, such as silica, etc., in the feed-water which has been brought into the boiler vessel onto the inside of the system but also discharging these components out the system by means of blowing-down.
The scale inhibition agent as referred to herein is one for preventing scaling of the hard component which has been brought into the water system, and for example, phosphates, such as trisodium phosphate, sodium tripolyphosphate, etc., or polymers, such as sodium polyacrylate, etc., are used.
Meanwhile, even in the case of adopting such a scale inhibition method, in the case where unexpected hard components leakage or the like occurs, and a scale deposition, the operation of a boiler is stopped, the boiler water is discharged by means of full blowing-down, and thereafter, chemical cleaning with a scale dissolving and removing agent is carried out. For example, PTL 1 describes a scale removal method by chemical cleaning with a chelating agent, such as high-concentration ethylenediaminetetraacetic acid (EDTA), etc., or an organic acid, such as sulfamic acid, etc.
However, according to the scale removal method of PTL 1, the boiler is once stopped, and hence, there are involved such problems that the productivity is impaired; and that the cleaning costs are separately generated.
In order to solve the foregoing problems, there is proposed a method for removing a scale without stopping the operation of the boiler. For example, PTL 2 describes a method in which a specified chelating agent, such as EDTA, nitrilotriacetic acid (NTA), diethylenetriamine, etc., and a specified dispersant, such a polymaleic acid, etc., are added into a boiler water, and a scale is removed while operating the boiler.